2018MNRAS.480.1263H

We analyse interferometric data obtained for Regulus with AMBER (Astronomical Multi-BEam combineR) at high spectral resolution (λ/δλ~12 000) across the Brγ spectral line. The study of the photocentre displacement allows us to constrain a large number of stellar parameters - equatorial radius R_eq, equatorial velocity V_eq, inclination i, rotation-axis position angle PA_rot and flattening - with an estimation of gravity-darkening coefficient β using previously published theoretical results. We use the Simulation Code of Interferometric-observations for ROtators and CirCumstellar Objects (SCIROCCO), a semi-analytical algorithm dedicated to fast rotators. We chose Regulus because it is a very well-known edge-on star, for which an alternative approach is needed to check the previously published results. Our analysis showed that a significant degeneracy of solution is present. By confronting the results obtained by differential interferometry with those obtained by conventional long-base interferometry, we obtain similar results (within the uncertainties), thereby validating our approach, where V_eq and i are found separately. From the photocentre displacement, we can independently deduce PA_rot. We use two minimization methods to restrict observed stellar parameters via a fast rotator model: a non-stochastic method (χ² fit) and a stochastic one (Markov Chain Monte Carlo method), in order to check whether the correct global minimum is achieved, particularly with respect to the degeneracies of the gravity-darkening parameter β, where we demonstrate, using a quantitative analysis of parameters, that estimation of β is easier for stars with an inclination angle of around 45°.